Excerpts from Books and Wikipedia
"As usual, it was the emperor’s pressing need for men and money that gave him his chance. In November 1700 the promise of a contingent of 8,000 Brandenburg soldiers for the war about to get under way over the Spanish succession, plus promises of support in imperial politics, extracted the necessary approval. On 18 January 1701 the Elector Frederick III announced that henceforth he would be “Frederick I King in Prussia”: “in Prussia” rather than “of Prussia” because West Prussia remained under Polish suzerainty.80 Prussia rather than Brandenburg was chosen for the title because it lay outside the Holy Roman Empire and was unquestionably a sovereign possession." [Blanning: Frederick the Great, Kindle, p. 23]